U.S. Pat. No. 5,431,631 granted to the same inventor of this application discloses a safety syringe allowing a retraction of a used needle assembly into the syringe cylinder for preventing its outward re-protrusion and also providing a hollow needle portion 21 which may be detachably mounted on a shank portion 22 outside the syringe cylinder 11.
However, when coupling the hollow needle portion 21 on said shank portion 22 of the needle assembly 2, the shank portion 22 may be free rotated within the sleeve portion 12 of the syringe means 1, thereby influencing a reliable coupling of the hollow needle portion 21 on the shank portion 22 and causing unstable mounting of the needle assembly on the syringe means.
The present inventor has found such a drawback and made an improvement for overcoming the free-rotation problem of the shank portion in the sleeve portion of the syringe means.